I was told in an Astronomy class I took that it's because Uranus collided with something in the forming of the solar system. I figure no one is ever going to really know why, unless we can observe the creation of another solar system. Uranus has a strong magnetic field too, but it isn't in the same position as its rotational axis... it's in a more "normal" position 60° from its rotation axis. This is weird because magnetic fields around planets are normally caused by a rotating “liquid” mantle around a solid metalic core. So Saturn and Jupiter are big enough to have a metallic hydrogen core which explains why they have strong magnetic fields. I don’t think anyone has explained yet why Uranus and Neptune do, since they aren't very dense. I’m sure the other physics peeps on the forum know way more about this stuffs.

simeonie its decided because all the other planets are the same Interestingly enough they all rotate on there axis in the same direction, apart from Venus which goes t’other way around... So the sun rises in the West and sets in the East. They used the same lame excuse for this that it must have collided with something... All the planets are in a similar orbital plane too apart from Pluto which is at a crazy angle. Pluto along with Neptune has a very elongated ellipsoidal orbit. So all the planets are similar but there is an exception to most rules.

Then there's Mercury, which rotates in a 3/2 gravitational resonance with its orbit. Yes, Venus rotates the wrong way. Earth is at a nominal 23 1/2 degrees, but it hasn't always been at this angle, and is predicted to eventually flip over in its "side" because of gravitational perturbations..

Anyway, the point is that every planet has a different story. Now we think there are ten different stories.

To me the several planets mentioned with strangely tilted axis suggests that we once had a close encounter of the 4th kind, a very large celestial body with massive gravitational forces (spelled S T A R). The planets would round out their orbits over time, but the direction of their axis would remain tilted from the encounter.

simeonie because it's very small and composed of mostly ice. There are lots of other objects that far out in the solar system making up the Kuiper Belt, which is a bit like the astroid belt but more spaced out and made up of mostly ice (not just water ice). Pluto has a very odd orbit too and so it's just as effective to call it a very large Kuiper Belt object... it does have a moon, but since the moon is of such large relative size it's debateable to say it's a planet. There are plenty of moons that are larger than Pluto out right, I think that includes most of the galilean system around Jupiter. There are even some moons bigger than Mercury, so it's all just a matter of opinion what you call a planet, a moon, an asteroid or a kuiper belt object.

quote:Well Could someone answer my very constructive question of what makes a planet a planet?

Apparently no, they can't!

Wikipedia says "An IAU committee which had already been working on a definition is now expected to promulgate one soon." Unless there are some astronomers out there with more up to date information or an informed opinion?

It also has some stuff on why Pluto's status is so contraversial (lots of rocks out there nearly as big, disproportionately large moon and eccentric orbit)

There is no strict all encompassing definition that I know, it’s to do with size, composition and location mostly. So if the thing is made up of rocky metals and found in the asteroid belt it's an asteroid. If it has a long period orbit, made out of ice, and has a vapour tail when close to the sun its a comet. Planets are distinguished by having systems of satellites or being bigger than other stuff, and being made of gas and ice further out in the solar system, and rocky metals within the inner solar system. Basically people found planets with telescopes first because of their size. Pluto is so far away and called a Planet because it was the first object found of sizeable note that far away. But since people have discovered other large objects that far away it’s under debate if Pluto should be relegated to “other stuff”.

At the moment pluto is still a planet, however it may not be for much longer.There a three proposals. And the IAU may decide in the next two weeks

The first one, comes down to size which if voted for would increase the number of planets , with Pluto being just one of several Kuiper Belt objects of about the same size that would qualify as a planet.

The second one, would remove pluto from the list. The definition says a planet is the dominant body in its immediate neighbourhood - a title based on its size relative to its neighbours and the dynamics of its orbit. Pluto would not count because it crosses the orbit of the larger Neptune.

The third but unlikly proposal. Under the third proposal there would be no planets. as they would be no longer be using that term to describe them? instead of planets they would be given a descriptive name comprised from what they were made from and there location.

Or there going back to the drawing board if no one agrees with any of the above.

I THOUGHT URANUS WAS A GAS GIANT AND AS SUCH SOLID THINGS WOULD PASS STRAIT THROUGH IT.I KNOW AFTER HALF A DOZEN CHARLIES AND A CHICKEN VINDALOO,MINE BECOMES A GAS GIANT.HOW DO YOU KNOW ITS NOT THE REST OF US ON OUR SIDE AND URANUS IS THE ONLY ONE THATS UPRIGHT.

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